(1) Field of the Invention
This invention relates to improvements in transmission systems for multi-wheel-drive (e.g. 4 WD) vehicles or the like.
(2) Brief Description of the Prior Art
The front and rear differentials of conventional 4 WD vehicles are connected together by the transfer case, when the vehicle is in 4 WD, and so they can only operate at the same speed. When the 4 WD vehicle negotiates a corner, the front wheels may have to roll up to e.g. 15% faster than the rear wheels because they follow a path of greater radius. This leads to strain (called "differential wind-up") between the two differentials and so the front and/or rear tires must slip to enable the vehicle to negotiate the corner. On soft surface e.g. sand, the effects of the tire slip may be minimal. However, on hard surfaces such as tarmac, the tire wear due to such slippage may be very marked.
When the vehicle is in 2 WD, the front differential is disconnected from the rear differential by the transfer case. However, the front wheels, through the front differential, drive the front propellor shaft and this increases the fuel consumption of the vehicle over conventional 2 WD vehicles.
A number of solutions have been proposed in the past to overcome these problems.
The first proposal has been to provide the 4 WD vehicle with a front differential having a higher (numerically lower) gear ratio than the rear differential e.g. 4.10:1 compared with 4.11:1. While this marginally reduces the problem when the vehicle is cornering, the front wheels being slightly driven faster than the rear wheels, the difference is not sufficient to equal the difference (e.g. of 15%) which may be required and the front and/or rear wheels must slip, to compensate for the ratio difference, when the vehicle is travelling straight ahead in 4 WD.
The second proposal, which may be used with the first, has been the use of "free-wheeling hubs" on the front wheels. When the hubs are disengaged, they disconnect the front wheels from the front axles and so the front differential and its propellor shaft are not driven when the vehicle is in 2 WD. However, when the hubs are re-engaged to provide drive for the front wheels, the problem of the requirement from the front and rear wheels re-emerges.
In addition, the hubs are often difficult to engage when the vehicle is bogged and they cannot be engaged and disengaged when the vehicle is moving.
A third proposal has been the provision of a third differential, incorporated in the transfer case, enabling the front and rear differentials to be driven at different speeds. Examples of transfer cases incorporating this third differential include those found in "Jeep" (trade mark) vehicles (under the trade mark "Quadra-Trac") and in "Range Rover" (trade mark) vehicles. To ensure that both differentials are driven, the third differential must be of the limited-slip type and/or have a differential lock. This arrangement is very expensive and cannot be fitted to the transfer cases of existing vehicle.